This invention relates to the preparation of adhesives in film form, commonly called "film adhesives", containing epoxide resins, and to their use.
Structural adhesive bonding is a well established procedure in the manufacture of aircraft, and similar industries. Adhesives commonly employed for this purpose are often used in the form of solid films, thereby eliminating difficulties which may occur when a liquid adhesive is used, such as evaporation of a solvent, loss of adhesive from the required site, and uneven distribution. Epoxide resins have a high reputation as adhesives and are often required in film form.
Conventionally, film adhesives are prepared by a technique involving evaporation of a solvent or by extrusion. In the former method, a resin composition is dissolved in a volatile solvent and the solution is poured onto a flat surface: the solvent is then allowed or caused to evaporate, leaving a film of the composition. In the latter method, the resin composition is heated to its melting point, extruded through a narrow slit, and then cooled or allowed to cool. While one or other of these methods is suitable for making a film adhesive from many kinds of resin, they are unsuitable for making a film adhesive out of an epoxide resin which is both substantially insoluble in volatile solvents and not readily fusible, such as one in the B-stage or an advanced resin of very high molecular weight. They are also unsuitable for use with compositions in which a latent curing effect is achieved by using discrete particles of a solid epoxide resin and discrete particles of a solid hardener, the resin and hardener not reacting together whilst the components are in particulate form but starting to cure as soon as the components are brought into intimate contact by being dissolved in a solvent or by being fused together. Extrusion methods also suffer from the disadvantage that the advancement of the resin which may result can shorten the shelf-life of the film and lead to the premature gelation of the resin composition. Use of solvents may introduce problems of toxicity, flammability, or pollution.
A method has now been found by which a film adhesive may be prepared from an epoxide resin without causing excessive advancement of that resin; the shelf-life of the film adhesive is thus dependent only upon the nature of the resin and hardener and not upon the mode of its manufacture.